1. Field of the Invention
The present invention relates to semiconductor fabrication and more particularly to a reticle to form photoresist patterns used in the lithography process.
2. Description of the Prior Art
Generally, a phase shift mask or a chromium mask have been used in the step-and-repeat reduction projection system which forms a method to form a contact hole in semiconductor devices. FIG. 1 is a cross-sectional view illustrating a method for fabricating the conventional chromium mask (retitle). As shown in FIG. 1, a chromium film 2 is formed on a transparent quartz substrate 1 and the chromium film 2 is coated with a photoresist film 3. To form patterns on the transparent quartz substrate 1, the photoresist film 3 is developed after the E-beam writing process is performed so that a plurality of parts of the photoresist film 3 are removed in accordance with a predetermined pattern designed by a manufacturer, exposing the chromium film 2. By doing so, the chromium patterns may be formed on the transparent quartz substrate 1.
FIG. 2 is a cross-sectional view illustrating an exposure process for forming a contact hole using the mask having the chromium patterns of FIG. 1. To give a brief explanation of the exposure process in the step-and-repeat reduction projection system, there is shown only the chromium patterns 3a, a wafer 7 and a photoresist film 8. It should be noted that a photoresist film 8 is formed on the wafer 7 having an uneven surface so that the thickness of the photoresist film 8 is not uniform.
Accordingly, this uneven surface of the wafer 7 may cause the photoresist film 8 to have the thickness difference between the thick area A and thin area B thereof. As a result, in the case where the light source applies exposure light to the photoresist film 8 through the reticle, it is very difficult to perform the exact exposure process suitable for the whole area. Since the thickness of the photoresist film 8 in the area A is thick, the critical area, which is exposed to the light source in the area A, may be narrower than that in the area B. Due to this decrease of the critical area, when the worst comes to the worst, the photoresist patterns may not be generated on the wafer 7. On the contrary, the critical area, which is exposed to the light source in the area B, may be broader than that in the area A. In this case, when the worst comes to the worst, the bulk effect may be generated by the connection between neighboring patterns.
Therefore, in the case where contact holes are formed by the above conventional photomask (or reticle), contact holes become much smaller so that they are not opened or they becomes much larger so that holes are connected to a neighboring hole, causing poor chromium patterns.